A crucial problem in biology is the mechanism by which common and general cytoskeletal elements are recruited into a diverse and dynamic array of functionally distinct structures. Part of the problem concerns the spatial and temporal regulation over the assembly and disassembly of the cytoskeleton. In addition, structures within the cell show directed motility dependent upon the activities of specific motor proteins moving over the cytoskeletal array. Our long range goals are directed at understanding the functions of one part of the cytoskeletal array, the microtubules. Microtubules are required in all eukaryotic cells, for many cellular processes, notably the segregation of chromosomes in mitosis and meiosis and for the congression of the progenomes during fertilization. Errors in these processes can have disastrous consequences for the cell. All the microtubules in yeast are organized by a single organizing center called the spindle pole body (SPB). The SPB plays a key role in microtubule function. However, little is known about the components of the SPB or how they might be functionally regulated. To address these problems we are studying the process of nuclear fusion in yeast conjugation. Nuclear fusion is a simple microtubule dependent process akin to fertilization. As a means to identifying functional components of the SPB we will use a variety of genetic techniques to identify the genes and proteins responsible for nuclear fusion. One of the genes known to be required for nuclear fusion, KAR1, encodes and essential component of the SPB. Using a variety of genetic and biochemical approaches, we aim to identify proteins that interact with the KAR1 protein since some of these should also be components of the SPB. Nuclear fusion requires that the nuclei move together in a microtubule dependent manner. Another of the genes required for nuclear fusion, KAR3, encodes a microtubule dependent motor protein which is also of critical importance in mitosis and meiosis. By a combination of genetic and biochemical analysis we aim to understand the specific functions of KAR3 and other related motor proteins in the complex movements of the chromosomes, the spindle and the nucleus in mitosis, meiosis and nuclear fusion.